For many years, conspiracy theorists have claimed that the government conducts top-secret chemical testing in the skies above us. As evidence, they point to "chemtrails" - actually ordinary airplane contrails, or condensation trails - that, it's claimed, have some sinister purpose.

In his conspiracy book "Above Top Secret," Jim Marrs notes that "No one in a position of authority will admit that they exist, much less who is responsible, and what purpose they may serve. Unlike many mysteries, this one is visible to anyone who cares to look up on the days that large jets weave narrow and continuous vapor/chemical trails through the sky."

It's odd that conspiracy theorists are so certain they exist but can't even agree on what, exactly, they are or what they do. Some say it's a sinister government mind-control experiment. Others say the trails are a form of weather control. Still others insist that experimental drugs are being tested on unsuspecting urban populations.

Hard evidence of the existence of these chemtrails has been elusive, but earlier this week a video surfaced that claims to provide proof. It shows a plane landing in a fog, with what are claimed to be jets of chemicals spewing from the wings.

"A pilot of a commercial airliner made a mistake that irrefutably PROVES the existence of ‘CHEMTRAILS'- by forgetting to turn them off before he landed! We have video of the plane landing while still spraying CHEMTRAILS as it hits the runway. This is the first empirical evidence to back-up claims made (by) people, smeared as ‘conspiracy-theorists,' who claimed airlines are being used by government to spray aerosols into the air without the knowledge or consent of the people being sprayed. With proof like this, the public now has legal standing to file lawsuits, utilize subpoenas and force discovery of evidence."

This is not the first time that someone has claimed to have found hard evidence of chemtrails. In "Above Top Secret," Marrs offers this evidence:

"One Louisiana TV station in late 2007 took upon itself the task of testing water captured under a crosshatch of aerial trails. According to investigative reporter Jeff Ferrell, ‘KSLA News 12 had the sample tested at a lab. The results: high level of barium, 6.8 parts per million, (ppm). That's more than three times the toxic level set by the Environmental Protection Agency.'"

"The actual video clearly shows 68.8 ug/L (micrograms per liter), or 68.8 ppb (parts per billion).... 68.8 millionths of a gram per liter corresponds to 68.8 parts per billion, (and) the reporter was off by a factor of 100 because he read the '68.8′ as '6.8.' Ferrell overestimated the amount of barium in the test report by a factor of 100.... The test result was not ‘three times the toxic level set by the EPA'; it was around 30 times less than the EPA's toxic limit."

So the alarming levels of barium that conspiracy theorist Jim Marrs cited as evidence of chemtrails was in fact a mistake created by a TV reporter's poor math skills.

What about the new video showing explosive proof of chemtrails? There are several problems with this theory - aside from the fact that no one has ever shown that chemtrails are real.

First of all, it seems very unlikely that passenger airlines at commercial airports like the one seen in the video would be used in top-secret military experiments. Any passenger seated in a dozen or more window seats behind the airplane wings would have a clear view of any supposed "spraying" going on - which kind of defeats the purpose of having a super-secret conspiracy spraying plan no one can know about.

Secondly - despite claims to the contrary - the video does not show anything being sprayed from the wings; instead the "mysterious" trail is merely created by the air disruption as the plane's wings fly through the fog.

The whole idea of an unknown chemical being sprayed that far above us doesn't make sense from a meteorological point of view. As Robert Carroll notes on his website The Skeptic's Dictionary:

"Any biological or chemical agents released at 25,000 feet or above would be absolutely impossible to control, making any measurement of effects on the ground nearly impossible.... Such an exercise would be pointless, unless you just wanted to pollute the atmosphere."

There's no evidence that so-called chemtrails are any different from ordinary airplane contrails. Indeed, given how visible and common the chemtrails supposedly are - and the fact that there are many thousands of private pilots flying everything from small, fixed-wing aircraft to hot air balloons to ultralights that could presumably collect samples of these mysterious chemtrails - the fact that no evidence of them has surfaced is remarkable.

Furthermore, there's no indication that all of these supposed chemtrails are actually doing anything: If the mysterious chemicals are some sort of mind controlling drug or mass experiment, it seems that whatever is being used is ineffective because people act the same with or without chemtrails overhead. It seems the only people chemtrails have an effect on is conspiracy theorists.